If you read some of the breakdowns here, some of the materials used aren't cheap...
I scored 23AA/23TS/24 PAT/24RC. Academic average was 99.6th percentile. but it was because I used and abused all of the material below:
Crack DAT PAT - 10 tests Ace edition: $130
Crack DAT Reading - 5 tests Ace edition: $60
DAT Destroyer + Math Destroyer: $200
Cliff AP Biology: $20
Kaplan Blue Book: $50
DAT Achiever, 3 tests: $60
Topscore, 3 tests: $60
That's $580.
Everything I bought on that list was essential for me to get that score, and if I was missing any one of those material, I wouldn't have done as well.
You could choose not to buy some material, but then you'd always be wondering what score you could have had.